I have some files which have added and committed to master branch before.
NOw, I've made some changes in some files,but before adding and committing these modifications, I just want to abandon whatever I've done and come back to my previous , peaceful, state !!

WARNING! git reset --hard and git checkout -- some files has no undo! Be careful!

The best thing to do is to stash what you don't think you need with

git stash -u

(the -u will include not yet tracked files as well as tracked ones). Now you are back to a clean state. In case you did have something important in there and realized later, you can inspect your stash to find it again. To get them back again, you can

git stash pop

to "erase" them again

git stash -u

Don't do anything without a safety net.

To do this on a subset of files, or parts of files, you can:

git add -A && git stash save --patch

Unfortunately patch doesn't work with untracked files for stash.

If all your changes involve just tracked files or you don't care about untracked files hanging around, you can skip the git add and just do this:

git stash save --patch

You will then be prompted for what you want to stash (effectively removing it). This way you can reset only certain changes if you wish - safely.

Any ignored files are untouched. To wipe them,

git clean -xdf

this is permanent, since they are ignored, it's not as important to have a backup of them.

It's unclear exactly what you want to do here. If your goal is to replace your last commit with a new commit, git add the files you want to update as usual and then git commit --amend. This will create a new commit whose parent is the current commit's parent. You can also accomplish this with git reset --soft HEAD^ followed by git commit.